The invention concerns a method and an arrangement for the measurement of the acid-base status of blood, which embraces the determination of the pH, the pCO.sub.2 and the base excess BE.
The metabolic processes in the human organism can take place without disturbance only within a relatively narrowly limited pH-range. For stabilization of this pH range, which lies, for example in blood plasma, between about 7.37 and 7.43, the organism has available a series of regulatory mechanisms based upon electrolytically active material systems, which through regulation of the amount of each of their particle fractions attempt to maintain the normal pH-value against disturbances to the normal metabolism or from pathological processes.
The two most important electrolytically active material systems are the CO.sub.2 -system on the one hand and the system of non-volatile bases, composed of phosphates and proteinates, on the other hand, which are active in the blood as pH-determining fractions. Both systems are coupled through the bicarbonate HCO.sub.3.sup.-, originating from CO.sub.2, i.e. the volatile base, which forms the largest particle fraction.
It is now indeed possible without more to conceive through measuring techniques the pH-value of the blood plasma, which is representative of the pH-value of the organism. However, it is very laborious to provide the individual contributions to the pH-value, which originate from the proteinate-phosphate system (referred to hereafter as the PP-system), the CO.sub.2 -system or the bicarbonate system. Correspondingly, it is difficult to determine the measured variables pH, pCO.sub.2 and the buffer base concentration BB or the base excess BE, standing in connection therewith, customarily designated as "acid-base status".
On account of the fundamental importance of the pH-value for the total metabolism, the acid-base status is a significant variable for diagnosis and choice of therapy.
The previous methods for determination of the acid-base status proceed from the law of mass action for the bicarbonate-carbonic acid system, as follows: ##EQU1## which leads, through taking of logarithms, to the equation ##EQU2## according to the Henderson-Hasselbalch equation, when instead of the undissociated carbonic acid, the CO.sub.2 -partial pressure, i.e. pCO.sub.2, is used.
Thereby a linear function is obtained, with logarithmic graduation of the coordinates, a linear function, which is representable in two dimensions. From this can be chosen either a pH-HCO.sub.3 -diagram with pCO.sub.2 as parameter, a pCO.sub.2 -HCO.sub.3.sup.- -diagram with pH as parameter, or a pH-pCO.sub.2 -diagram with HCO.sub.3.sup.- as parameter.
If one chooses, for example, the pH-pCO.sub.2 -diagram, then there results from each two pH-pCO.sub.2 value pairs a line which belongs to a fixed HCO.sub.3 -value.
For the most part, in practice, the bicarbonate is not considered alone, but together with the proteinate-phosphate-system (PP). Both fractions together are customarily designated as "buffer bases" (BB).
To illustrate, FIG. 1 represents a diagram according to Astrup & Siggaard-Andersen, The Acid-Base Status of the Blood, Munksgaard, Copenhagen, 1974). There, the points A and B in the diagram are determined through equilibration of a blood sample with two CO.sub.2 -partial pressures corresponding to points A and B and through measurement of the attendant pH-value. The thereby determined titration line--also referred to as "equilibration line"--intersects the curve (BB) obtained through standardization. From the point of intersection can be read that in the blood sample, in addition to the pH-value determining CO.sub.2 (variable along the equilibration line), a concentration of 37 milli-equivalents per liter of buffer bases is present. (Normal value: about 48.0 milliequivalents per liter; altering the content of buffer bases displaces the equilibration line roughly parallel.)
From the titration line, moreover, the actual pCO.sub.2 can yet be found through measurement of the actual pH-value.
The cost of the measuring techniques for obtaining the value of the buffer bases is thus correctly high: one must prepare two precisely determined CO.sub.2 -gas-mixtures, and the blood sample must be held under physiological conditions, i.e. thermostatically controlled to avoid pH-thermodrift of the blood. Hemolysis must be excluded, since otherwise the erythrocytes, having a pH-value lying about 0.2 pH-units below the plasma-pH-value, alter the plasma-pH-value and thereby the measured values.
Likewise, the so-called "direct method" (Siggaard-Andersen, The Acid-Base Status of the Blood, Munksgaard, Copenhagen, 1974) is laborious.